Field of the Invention
The invention relates to a hearing instrument comprising a T-coil antenna for picking up a base-band audio signal from a transducer coil of a telephone device and a functional unit operating in a duty cycling mode.
Description of Related Art
Such T-coil antenna of a hearing instrument is a ferrite coil which is sensitive not only to the base band audio signal to be picked up from the telephone device transducer, but is also quite sensitive to all other kinds of magnetic fields as induced by noise sources; such noise sources may be external or internal with regard to the hearing instrument. Examples of such noise sources are all power sinks which are duty cycled and which typically require a source current with an AC component having frequencies from a few dozen to a few thousand Hertz. Typically, such AC content is reduced in hearing instruments by either load balancing or noise shaping or large bulk capacitors.
Hearing instruments may be provided with radio frequency (RF) transceiver units for wireless communication purposes. Such RF transceivers, which typically operate around 2.4 GHz, work in a duty cycled mode as they transmit and/or receive RF packets only during a limited time. Load balancing and noise shaping approaches can be applied to such RF transceivers only to a very limited extent, since such approaches, due to the significant peak currents drawn by the RF transceiver, would make operation of the transceiver un-economical or even may hamper the functionality of the transceiver.
Thus, peaks of the current drawn by the transceiver may induce a magnetic field which, in turn, may be picked up by the T-coil antenna, resulting in an audible distortion of the audio signal. Although the transceiver typically will not be used for audio signal transmission/reception when the T-coil antenna is active, the transceiver may be used for other purposes than audio signal transmission, such as for remote control purposes, during times when the T-coil antenna is active, albeit with a low duty cycle of, for example, 1%. Thus, even when the T-coil antenna is used without a concurrent RF audio link, the RF transceiver may induce a regular click signal in the audio signal picked up by the T-coil antenna, such as every 100 ms, which may be audible to the hearing instrument user.
U.S. Pat. No. 7,529,378 B2 relates to a hearing instrument which is adapted to suppress interfering signals, such as GSM signals, by detecting the repetition rate or frequency of the interfering signal by analyzing the audio signal picked up by the hearing instrument microphone and by attenuating the audio signal at the expected next point in time of occurrence of the interfering signal.
International Patent Application Publication WO 2009/108553 A1 and corresponding U.S. Pat. No. 7,529,378 B2 relates to a hearing instrument which is adapted to suppress interfering U.S. Pat. No. 8,340,333 relate to a hearing instrument comprising a T-coil, wherein the input audio signal is analyzed in order to create a waveform of a noise signal in the input audio signal, and wherein the waveform of the noise signal is subtracted from the input audio signal in order to remove noise from the audio signal.
U.S. Patent Application Publication 2011/0300874 A1 relates to a method for removing TDMA noise from an audio signal, wherein the TDMA noise filter only is activated during non-speech intervals of the audio signal.
German Patent Application DE 10 2009 018 425 A1 relates to a hearing instrument comprising a T-coil and a transmission coil for wireless communication above the frequency range of the telephone signal, wherein the audio signal picked up by the T-coil is analyzed in order to remove noise resulting from the transmission coil by subtracting a model noise signal or by adding an anti-noise signal.